Phase-change memory and method of forming same

ABSTRACT

A device and a method of forming the same are provided. The device includes a substrate, a first dielectric layer over the substrate, a bottom electrode extending through the first dielectric layer, a first buffer layer over the bottom electrode, a phase-change layer over the first buffer layer, a top electrode over the phase-change layer, and a second dielectric layer over the first dielectric layer. The second dielectric layer surrounds the phase-change layer and the top electrode. A width of the top electrode is greater than a width of the bottom electrode.

BACKGROUND

Phase change technology is promising for next generation memories. It uses chalcogenide semiconductors for storing states. The chalcogenide semiconductors, also called phase change materials, have a crystalline state and an amorphous state. In the crystalline state, the phase change materials have a low resistivity, while in the amorphous state they have a high resistivity. The resistivity ratios of the phase change materials in the amorphous and crystalline states are typically greater than 1000 and thus the phase change memory devices are unlikely to have erroneous reading. The chalcogenide materials are stable at certain temperature ranges in both crystalline and amorphous states and can be switched back and forth between the two states by electric pulses. One type of memory device that uses the principal of phase change in chalcogenide semiconductors is commonly referred to as phase-change random access memory (PCRAM).

PCRAM has several operating and engineering advantages, including high speed, low power, non-volatility, high density, and low cost. For example, PCRAM devices are non-volatile and may be written into rapidly, for example, within less than about 50 nanoseconds. The PCRAM cells may have a high density. In addition, PCRAM memory cells are compatible with CMOS logic and can generally be produced at a low cost compared to other types of memory cells.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1-4 illustrate cross-sectional views of intermediate stages in the manufacturing of a phase-change random access memory (PCRAM) cell in accordance with some embodiments.

FIG. 5 illustrates a cross-sectional view of a PCRAM cell in accordance with some embodiments.

FIG. 6 illustrates a cross-sectional view of a PCRAM cell in accordance with some embodiments.

FIG. 7 illustrates a cross-sectional view of a PCRAM cell in accordance with some embodiments.

FIGS. 8-13 illustrate cross-sectional views of intermediate stages in the manufacturing of a PCRAM cell in accordance with some embodiments.

FIG. 14 illustrates a cross-sectional view of a PCRAM cell in accordance with some embodiments.

FIG. 15 is a flow diagram illustrating a method of forming a PCRAM cell in accordance with some embodiments.

FIG. 16 is a flow diagram illustrating a method of forming a PCRAM cell in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

Embodiments will be described with respect to a specific context, namely, a semiconductor device, such a phase-change random access memory (PCRAM) device, and a method of forming the same. By forming buffer layers between top and bottom electrodes and a phase-change material layer, metal diffusion from the top and bottom electrodes into the phase-change material layer may be reduced or avoided. By avoiding the metal contamination in the phase-change material layer, read/write cycling endurance of a PCRAM device may be improved.

FIGS. 1-4 illustrate cross-sectional views of intermediate stages in the manufacturing of a PCRAM cell 100 in accordance with some embodiments. In some embodiments, the PCRAM cell 100 comprises a substrate 101. The substrate 101 may comprise, for example, bulk silicon, doped or undoped, or an active layer of a semiconductor-on-insulator (SOI) substrate. Generally, an SOI substrate comprises a layer of a semiconductor material, such as silicon, formed on an insulator layer. The insulator layer may be, for example, a buried oxide (BOX) layer or a silicon oxide layer. The insulator layer is provided on a substrate, such as a silicon or glass substrate. Alternatively, the substrate 101 may include another elementary semiconductor, such as germanium; a compound semiconductor including silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof. Other substrates, such as multi-layered or gradient substrates, may also be used.

In some embodiments, an access transistor 103 is formed over the substrate 101. The access transistor 103 includes a gate stack comprising a gate dielectric 105 and a gate electrode 107, spacers 109 on opposite sidewalls of the gate stack, and source/drain regions 111 adjacent to the respective spacers 109. For simplicity, components that are commonly formed in integrated circuits, such as gate silicides, source/drain silicides, contact etch stop layers, and the like, are not illustrated. In some embodiments, the access transistor 103 may be formed using any acceptable methods. In some embodiments, the access transistor 103 may be a planar MOSFET, a FinFET, or the like.

In some embodiments, additional active and/or passive devices may be formed on the substrate 101. The one or more active and/or passive devices may include transistors, capacitors, resistors, diodes, photo-diodes, fuses, or the like. The one or more active and/or passive devices may be formed using any acceptable methods. One of ordinary skill in the art will appreciate that the above examples are provided for the purpose of illustration only and are not meant to limit the present disclosure in any manner. Other circuitry may be also used as appropriate for a given application.

In some embodiments, an interconnect structure 113 is formed over the access transistor 103 and the substrate 101. The interconnect structure 113 may comprise one or more metallization layers 115 ₀ to 115 _(M), wherein M+1 is the number of the one or more metallization layers 115 ₀ to 115 _(M). In some embodiments, the value of M may vary according to design specifications. In some embodiments, the metallization layer 115 _(M) may be an intermediate metallization layer of the interconnect structure 113. In such embodiments, further metallization layers are formed over the metallization layer 115 _(M). In some embodiments, M is equal to 1. In other embodiments, M is greater than 1.

In some embodiments, the one or more metallization layers 115 ₀ to 115 _(M), comprise one or more dielectric layers 117 ₀ to 117 _(M), respectively. The dielectric layer 117 ₀ is an inter-layer dielectric (ILD) layer, and the dielectric layers 117 ₁ to 117 _(M) are inter-metal dielectric (IMD) layers. The ILD layer and the IMD layers may include low-k dielectric materials having k values, for example, lower than about 4.0 or even 2.0. In some embodiments, the ILD layer and IMD layers may be made of, for example, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), fluorosilicate glass (FSG), SiOxCy, Spin-On-Glass, Spin-On-Polymers, silicon carbon material, compounds thereof, composites thereof, combinations thereof, or the like, formed by any suitable method, such as spin-on coating, chemical vapor deposition (CVD), plasma-enhanced CVD (PECVD), atomic layer deposition (ALD), a combination thereof, or the like.

In some embodiments, etch stop layers (ESLs) 123 ₁ to 123 _(M) are formed between adjacent ones of the dielectric layers 117 ₀ to 117 _(M). A material for the ESLs 123 ₁ to 123 _(M) is chosen such that etch rates of the ESLs 123 ₁ to 123 _(M) are less then etch rates of corresponding ones of the dielectric layers 117 ₁ to 117 _(M). In some embodiments, an etching process that etches the dielectric layers 117 ₁ to 117 _(M) faster than the ESLs 123 ₁ to 123 _(M) is a dry etching process performed using an etchant comprising a C_(x)F_(y)-based gas, or the like. In some embodiments, an etch rate of the ESL 123 _(K) is less than an etch rate of the dielectric layer 117 _(K) (with K=1, . . . , M). In some embodiments, each of the ESLs 123 ₁ to 123 _(M) may comprise one or more layers of dielectric materials. Suitable dielectric materials may include oxides (such as silicon oxide, aluminum oxide, or the like), nitrides (such as SiN, or the like), oxynitrides (such as SiON, or the like), oxycarbides (such as SiOC, or the like), carbonitrides (such as SiCN, or the like), carbides (such as SiC, or the like), combinations thereof, or the like, and may be formed using spin-on coating, CVD, PECVD, ALD, a combination thereof, or the like.

In some embodiments, the metallization layer 115 ₀ further comprises conductive plugs 121 ₀ within the dielectric layer 117 ₀, and the metallization layers 115 ₁ to 115 _(M) further comprise one or more conductive interconnects, such as conductive lines 119 ₁ to 119 _(M) and conductive vias 121 ₁ to 121 _(M), within the dielectric layers 117 ₁ to 117 _(M), respectively. The conductive plugs 121 ₀ electrically couple the source/drain regions 111 and the gate electrode 107 of the access transistor 103 to the conductive lines 119 ₁ to 119 _(M) and the conductive vias 121 ₁ to 121 _(M).

In some embodiments, the conductive plugs 121 ₀, the conductive lines 119 ₁ to 119 _(M) and the conductive vias 121 ₁ to 121 _(M) may be formed using any suitable method, such as a damascene method, a dual damascene method, or the like. In some embodiments, the steps for forming the conductive plugs 121 ₀, the conductive lines 119 ₁ to 119 _(M) and the conductive vias 121 ₁ to 121 _(M) include forming openings in the respective dielectric layers 117 ₀ to 117 _(M), depositing one or more barrier/adhesion layers (not explicitly shown) in the openings, depositing seed layers (not explicitly shown) over the one or more barrier/adhesion layers, and filling the openings with a conductive material (not explicitly shown). A chemical mechanical polishing (CMP) is then performed to remove excess materials of the one or more barrier/adhesion layers, the seed layers, and the conductive material overfilling the openings. In some embodiments, topmost surfaces of the conductive plugs 121 ₀ are substantially coplanar or level with a topmost surface of the dielectric layer 117 ₀ within process variations of the CMP process. In some embodiments, topmost surfaces of the conductive lines 119 ₁ to 119 _(M) are substantially coplanar or level with topmost surface of the dielectric layers 117 ₁ to 117 _(M), respectively, within process variations of the CMP process.

In some embodiments, the one or more barrier/adhesion layers may comprise titanium, titanium nitride, tantalum, tantalum nitride, a combination thereof, a multilayer thereof, or the like, and may be formed using physical vapor deposition (PVD), CVD, ALD, a combination thereof, or the like. The one or more barrier/adhesion layers protect the respective dielectric layers 117 ₀ to 117 _(M) from diffusion and metallic poisoning. The seed layers may comprise copper, titanium, nickel, gold, manganese, a combination thereof, a multilayer thereof, or the like, and may be formed by ALD, CVD, PVD, sputtering, a combination thereof, or the like. The conductive material may comprise copper, aluminum, tungsten, combinations thereof, alloys thereof, multilayers thereof, or the like, and may be formed using, for example, by plating, or other suitable methods.

Referring further to FIG. 1, a dielectric layer 125 is formed over the metallization layer 115 _(M). In some embodiments, the dielectric layer 125 may be formed using similar materials and methods as the dielectric layers 117 ₀ to 117 _(M) and the description is not repeated herein. In some embodiments, the dielectric layer 125 may have a thickness between about 40 nm and about 80 nm. Such a range of the thickness of the dielectric layer 125 allows for integrating processes for forming the PCRAM cell 100 with logic processes. In some embodiments, the dielectric layer 125 is patterned to form an opening 127 in the dielectric layer 125. The patterning process may include suitable photolithography and etching methods. In some embodiments, the opening 127 exposes underlying conductive line 119 _(M). In some embodiments, the opening 127 has a width W₁ between about 5 nm and about 40 nm. Such a range of the width of the opening 127 allows for reducing a write current and a write power of the PCRAM cell 100.

Referring to FIG. 2, a conductive layer 201 is formed in the opening 127 (see FIG. 1). In some embodiments, the conductive layer 201 may comprise a conductive material such as Ti, Co, W, Ru, C, WN, TiN, TiW, TiAl, TiAlN, a combination thereof, a multilayer thereof, or like, and may be formed using CVD, ALD, PVD, a combination thereof, or the like. In some embodiments, the conductive material is deposited in the opening 127 and may overfill the opening 127. In some embodiments, a planarization process, such as a CMP process, an etching process, a grinding process, a combination thereof, or the like, is performed on the conductive material to remove excess portions of the conductive material overfilling the opening 127. In such embodiments, a topmost surface of the conductive layer 201 substantially coplanar or level with a topmost surface of the dielectric layer 125 within process variations of the planarization process. The conductive layer 201 may also be referred to as a bottom electrode. The conductive layer 201 has a width that is equal to the width W₁ of opening 127 (see FIG. 1).

In some embodiments, a buffer material 203 is blanked deposited over the dielectric layer 125 and the conductive layer 201. The buffer material 203 may comprise amorphous carbon, W, WN, TaN, Ru, graphene, MoS₂, WS₂, a combination thereof, a multilayer thereof, or the like, and by formed using CVD, ALD, PVD, a combination thereof, or the like. In some embodiments, the buffer material 203 has a thickness between about 1 nm and about 10 nm.

In some embodiments, a phase-change material 205 is blanked deposited over the buffer material 203. The phase-change material 205 may comprise chalcogenide materials including one or more of Ge, Te, and Sb, e.g., which may be GeSbTe, or stoichiometric materials. The phase-change material 205 may be formed using ALD, CVD, PECVD, a combination thereof, or the like. In some embodiments, the phase-change material 205 has a thickness between about 20 nm and about 100 nm. Such a range of the thickness of the phase-change material 205 allows for integrating processes for forming the PCRAM cell 100 with logic processes.

In some embodiments, a buffer material 207 is blanked deposited over the phase-change material 205. The buffer material 207 may be formed using similar materials and methods as the buffer material 203 and the description is not repeated herein. In some embodiments, the buffer material 203 and the buffer material 207 comprise a same material. In other embodiments, the buffer material 203 and the buffer material 207 comprise different materials. In some embodiments, the buffer material 207 has a thickness between about 1 nm and about 10 nm.

In some embodiments, a conductive material 209 is blanket deposited over the buffer material 207. In some embodiments, the conductive material 209 may be formed using similar materials and methods as the conductive layer 201 and the description is not repeated herein. In some embodiments, the conductive layer 201 and the conductive material 209 may comprise a same material. In other embodiments, the conductive layer 201 and the conductive material 209 may comprise different materials. In some embodiments, the conductive material 209 has a thickness between about 10 nm and about 20 nm.

Referring to FIG. 3, the buffer material 203, the phase-change material 205, the buffer material 207 and the conductive material 209 (see FIG. 2) are patterned, such that the conductive layer 201 and remaining portions of the buffer material 203, the phase-change material 205, the buffer material 207 and the conductive material 209 form a PCRAM structure 309. In some embodiments, the buffer material 203, the phase-change material 205, the buffer material 207 and the conductive material 209 may be patterned using suitable photolithography and etching methods. In some embodiments, a single mask may be used to pattern the buffer material 203, the phase-change material 205, the buffer material 207 and the conductive material 209. The suitable etching processes may comprise one or more dry etching processes, one or more wet etching processes, a combination thereof, or the like. In some embodiments, the buffer material 203, the phase-change material 205, the buffer material 207 and the conductive material 209 are patterned using a single etch process, which may be a physical etch process or a chemical etch process. In other embodiments, the buffer material 203, the phase-change material 205, the buffer material 207 and the conductive material 209 are patterned using two different etch processes, with the first etch process patterning the conductive material 209 and the second etch process patterning the buffer material 203, the phase-change material 205 and the buffer material 207. In some embodiments, the first etch process may be a chemical etch process and the second etch process may be a physical etch process. By using the physical etch process, etch damage of the phase-change material 205 is reduced or avoided. In other embodiments, the first etch process may be a first chemical etch process and the second etch process may be a second chemical etch process. The first chemical etch process may be performed using Cl-containing etchants. The second chemical etch process may be performed using suitable etchants without including Cl-containing etchants. By not using Cl-containing etchants in the second chemical etch, etch damage of the phase-change material 205 is reduced or avoided.

In some embodiments, a remaining portion of the buffer material 203 forms a buffer layer 301, a remaining portion of the phase-change material 205 forms a phase-change layer 303, a remaining portion of the buffer material 207 forms a buffer layer 305, and a remaining portion of the conductive material 209 forms a conductive layer 307, such that the PCRAM structure 309 comprises the conductive layer 201, the buffer layer 301 over the conductive layer 201, the phase-change layer 303 over the buffer layer 301, the buffer layer 305 over the phase-change layer 303, and the conductive layer 307 over the buffer layer 305. The conductive layer 307 may also be referred to as a top electrode. In some embodiments, each of the buffer layer 301, the phase-change layer 303, the buffer layer 305, and the conductive layer 307 has a width W₂ between about 5 nm and about 200 nm. In some embodiments, the width of the conductive layer 201 is less than the width W₂.

Referring to FIG. 4, a dielectric layer 401 is formed over the dielectric layer 125 and surrounding the PCRAM structure 309. In some embodiments, the dielectric layer 401 may be formed using similar material and methods as the dielectric layer 125 described above with reference to FIG. 1 and the description is not repeated herein. In some embodiments, the dielectric layer 401 is formed by depositing a suitable dielectric material over the dielectric layer 125 and the PCRAM structure 309 and performing a planarization process on the dielectric material to remove portions of the dielectric material extending above a topmost surface of the PCRAM structure 309. The planarization process may comprise a CMP process, an etching process, a grinding process, a combination thereof, or the like. After performing the planarization process, a topmost surface of the dielectric layer 401 is substantially coplanar or level with the topmost surface of the PCRAM structure 309 within process variations of the planarization process.

Subsequently, additional metallization layers 115 _(M+1) to 115 _(M+N) are formed over the dielectric layer 401 and the PCRAM structure 309, with the metallization layer 115 _(M+N) being the last metallization layer of the interconnect structure 113. In some embodiments, the dielectric layers 117 _(M+X) (with X=1, . . . , N) may be formed using similar materials and methods as the dielectric layers 117 ₀ to 117 _(M) described above with reference to FIG. 1, and the description is not repeated herein. In some embodiments, the ESLs 123 _(M+X) (with X=1, . . . , N) may be formed using similar materials and methods as the ESLs 123 ₁ to 123 _(M) described above with reference to FIG. 1, and the description is not repeated herein. In some embodiments, the conductive lines 119 _(M+X) (with X=1, . . . , N) may be formed using similar materials and methods as the conductive lines 119 ₁ to 119 _(M) described above with reference to FIG. 1, and the description is not repeated herein. In some embodiments, the conductive vias 121 _(M+X) (with X=1, . . . , N) may be formed using similar materials and methods as the conductive vias 121 ₁ to 121 _(M) described above with reference to FIG. 1, and the description is not repeated herein. In some embodiments, N is equal to 1. In other embodiments, N is greater than 1.

Referring further to FIG. 4, by forming the buffer layer 301 between the conductive layer 201 and the phase-change layer 303, metal diffusion from the conductive layer 201 into the phase-change layer 303 may be reduced or avoided. In some embodiments where the conductive layer 201 comprises TiN, the buffer layer 301 prevents Ti atoms from diffusing into the phase-change layer 303. By forming the buffer layer 305 between the conductive layer 307 and the phase-change layer 303, metal diffusion from the conductive layer 307 into the phase-change layer 303 may be reduced or avoided. In some embodiments where the conductive layer 307 comprises TiN, the buffer layer 305 prevents Ti atoms from diffusing into the phase-change layer 303. In some embodiments, by avoiding the metal contaminants in the phase-change layer 303, read/write cycling endurance of the PCRAM structure 309 may be improved. In some embodiments, the conductive layers 201 and 307 comprise TiN and the buffer layers 301 and 305 comprise amorphous carbon. In other embodiments, the conductive layer 201 comprises Ru, the conductive layer 307 comprises TiN, and the buffer layers 301 and 305 comprise amorphous carbon. In yet other embodiments, the conductive layer 201 comprises TiN, the conductive layer 307 comprises Ru, and the buffer layers 301 and 305 comprise amorphous carbon.

FIG. 5 illustrates a cross-sectional view of a PCRAM cell 500 in accordance with some embodiments. In some embodiments, the PCRAM cell 500 may be similar to the PCRAM cell 100 illustrated in FIG. 4, with similar features of the PCRAM cell 500 and the PCRAM cell 100 being labeled with similar numerical references and descriptions of the similar features are not repeated herein. In some embodiments, the PCRAM cell 500 may be formed using process steps described above with reference to FIG. 1-4, but omitting the formation of the buffer material 203. In distinction with the PCRAM structure 309 of the PCRAM cell 100 (see FIG. 1), the buffer layer 301 is omitted in the PCRAM structure 501 of the PCRAM cell 500 and the PCRAM structure 501 comprises the conductive layer 201 in physical contact with phase-change layer 303. By forming the buffer layer 305 between the conductive layer 307 and the phase-change layer 303, metal diffusion from the conductive layer 307 into the phase-change layer 303 may be reduced or avoided. In some embodiments, the metal diffusion from the conductive layer 201 into the phase-change layer 303 may be reduced or avoided by forming the conductive layer 201 with a conductive material comprising metal elements with reduced diffusion coefficients. In such embodiments, the conductive layer 201 may comprise Ru, W, WN, C, a combination thereof, a multilayer thereof, or the like.

FIG. 6 illustrates a cross-sectional view of a PCRAM cell 600 in accordance with some embodiments. In some embodiments, the PCRAM cell 600 may be similar to the PCRAM cell 100 illustrated in FIG. 4, with similar features of the PCRAM cell 600 and the PCRAM cell 100 being labeled with similar numerical references and descriptions of the similar features are not repeated herein. In some embodiments, the PCRAM cell 600 may be formed using process steps described above with reference to FIG. 1-4, but omitting the formation of the buffer material 207. In distinction with the PCRAM structure 309 of the PCRAM cell 100 (see FIG. 1), the buffer layer 305 is omitted in the PCRAM structure 601 of the PCRAM cell 600 and the PCRAM structure 601 comprises the conductive layer 307 in physical contact with the phase-change layer 303. By forming the buffer layer 301 between the conductive layer 201 and the phase-change layer 303, metal diffusion from the conductive layer 201 into the phase-change layer 303 may be reduced or avoided. In some embodiments, the metal diffusion from the conductive layer 307 into the phase-change layer 303 may be reduced or avoided by forming the conductive layer 307 with a conductive material comprising metal elements with reduced diffusion coefficients. In such embodiments, the conductive layer 307 may comprise Ru, W, WN, C, a combination thereof, a multilayer thereof, or the like.

FIG. 7 illustrates a cross-sectional view of a PCRAM cell 700 in accordance with some embodiments. In some embodiments, the PCRAM cell 700 may be similar to the PCRAM cell 100 illustrated in FIG. 4, with similar features of the PCRAM cell 700 and the PCRAM cell 100 being labeled with similar numerical references and descriptions of the similar features are not repeated herein. In some embodiments, the PCRAM cell 700 may be formed using process steps described above with reference to FIG. 1-4, but omitting the formation of the buffer materials 203 and 207. In distinction with the PCRAM structure 309 of the PCRAM cell 100 (see FIG. 1), the buffer layers 301 and 305 are omitted in the PCRAM structure 701 of the PCRAM cell 700 and the PCRAM structure 701 comprises the conductive layers 201 and 307 in physical contact with the phase-change layer 303. In some embodiments, the metal diffusion from the conductive layers 201 and 307 into the phase-change layer 303 may be reduced or avoided by forming the conductive layers 201 and 307 with a conductive material comprising metal elements with reduced diffusion coefficients. In some embodiments, the conductive layer 201 may comprise Ru, W, WN, C, a combination thereof, a multilayer thereof, or the like. In other embodiments, the conductive layer 307 may comprise Ru, W, WN, C, a combination thereof, a multilayer thereof, or the like. In yet other embodiments, each of the conductive layers 201 and 307 may comprise Ru, W, WN, C, a combination thereof, a multilayer thereof, or the like.

FIGS. 8-13 illustrate cross-sectional views of intermediate stages in the manufacturing of a PCRAM cell 800 in accordance with some embodiments. In some embodiments, the structure illustrated in FIG. 8 is similar to the structure illustrated in FIG. 1, with similar features being labeled with similar numerical references and descriptions of the similar features are not repeated herein. In some embodiments, the structure illustrated in FIG. 8 may be formed using process steps described above with reference to FIG. 1 and the description is not repeated herein.

Referring to FIG. 9, a conductive layer 901 is formed in the opening 127 (see FIG. 8). In some embodiments, the conductive layer 901 may be formed using similar materials and methods as the conductive layer 201 described above with reference to FIG. 2 and the description is not repeated herein. The conductive layer 901 may be also referred to as a bottom electrode.

Referring to FIG. 10, the conductive layer 901 is recessed below a topmost surface of the dielectric layer 125 to a depth D₁ to form a recess 1001. In some embodiments, the conductive layer 901 may be recessed using a suitable etching process. In some embodiments, the suitable etching process may be a chemical etch process. In some embodiments, the depth D₁ is between about 5 nm and about 30 nm.

Referring to FIG. 11, a buffer layer 1101 is formed over the conductive layer 901 in the recess 1001 (see FIG. 10). In some embodiments, a buffer material is deposited in the recess 1001 and may overfill the recess 1001. In some embodiments, a planarization process, such as a CMP process, an etching process, a grinding process, a combination thereof, or the like, is performed on the buffer material to remove excess portions of the buffer material overfilling the recess 1001. In such embodiments, a topmost surface of the buffer layer 1101 is substantially coplanar or level with a topmost surface of the dielectric layer 125 within process variations of the planarization process. In some embodiments, the buffer material of the buffer layer 1101 may be formed using similar materials and methods as the buffer material 203 described above with reference to FIG. 2 and the description is not repeated herein. In some embodiments, the buffer layer 1101 has a thickness between about 1 nm and about 10 nm. In some embodiments, the buffer layer 1101 has a width between about 5 nm and about 40 nm. In some embodiments, the buffer layer 1101 and the conductive layer 901 have a same width.

In some embodiments, after forming the buffer layer 1101, a phase-change material 205 is blanket deposited over the buffer layer 1101 and the dielectric layer 125, a buffer material 207 is blanket deposited over the phase-change material 205, and a conductive material 209 is blanket deposited over the buffer material 207. In some embodiments, the phase-change material 205, the buffer material 207, and the conductive material 209 may be formed as described above with reference to FIG. 2 and the description is not repeated herein. In some embodiments, the buffer layer 1101 and the buffer material 207 comprise a same material. In other embodiments, the buffer layer 1101 and the buffer material 207 comprise different materials. In some embodiments, the conductive layer 901 and the conductive material 209 may comprise a same material. In other embodiments, the conductive layer 901 and the conductive material 209 may comprise different materials.

Referring to FIG. 12, the phase-change material 205, the buffer material 207 and the conductive material 209 (see FIG. 11) are patterned, such that the conductive layer 901, the buffer layer 1101 and remaining portions of the phase-change material 205, the buffer material 207 and the conductive material 209 form a PCRAM structure 1207. In some embodiments, the phase-change material 205, the buffer material 207 and the conductive material 209 may be patterned using suitable photolithography and etching methods. In some embodiments, a single mask may be used to pattern the phase-change material 205, the buffer material 207 and the conductive material 209. The suitable etching processes may comprise one or more dry etching processes, one or more wet etching processes, a combination thereof, or the like. In some embodiments, the phase-change material 205, the buffer material 207 and the conductive material 209 are patterned using a single etch process, which may be a physical etch process or a chemical etch process. In other embodiments, the phase-change material 205, the buffer material 207 and the conductive material 209 are patterned using two different etch processes, with the first etch process patterning the conductive material 209 and the second etch process patterning the phase-change material 205 and the buffer material 207. In some embodiments, the first etch process may be a chemical etch process and the second etch process may be a physical etch process. By using the physical etch process, etch damage of the phase-change material 205 is reduced or avoided. In other embodiments, the first etch process may be a first chemical etch process and the second etch process may be a second chemical etch process. The first chemical etch process may be performed using Cl-containing etchants. The second chemical etch process may be performed using suitable etchants without including Cl-containing etchants. By not using Cl-containing etchants in the second chemical etch, etch damage of the phase-change material 205 is reduced or avoided.

In some embodiments, a remaining portion of the phase-change material 205 forms a phase-change layer 1201, a remaining portion of the buffer material 207 forms a buffer layer 1203, and a remaining portion of the conductive material 209 forms a conductive layer 1205, such that the PCRAM structure 1207 comprises the conductive layer 901, the buffer layer 1101 over the conductive layer 901, the phase-change layer 1201 over the buffer layer 1101, the buffer layer 1203 over the phase-change layer 1201, and the conductive layer 1205 over the buffer layer 1203. The conductive layer 1205 may also be referred to as a top electrode. In some embodiments, each of the phase-change layer 1201, the buffer layer 1203, and the conductive layer 1205 has a width W₃ between about 5 nm and about 200 nm. In some embodiments, the width of the conductive layer 901 is less than the width W₃. In some embodiments, the width of the buffer layer 1101 is less than the width W₃.

Referring to FIG. 13, a dielectric layer 401 is formed over the dielectric layer 125 and surrounding the PCRAM structure 1207. In some embodiments, the dielectric layer 401 is formed using process steps described above with reference to FIG. 4 and the description is not repeated herein. Subsequently, additional metallization layers 115 _(M+1) to 115 _(M+N) are formed over the dielectric layer 401 and the PCRAM structure 1207, with the metallization layer 115 _(M+N) being the last metallization layer of the interconnect structure 113. In some embodiments, the metallization layers 115 _(M+1) to 115 _(M+N) are formed using process steps described above with reference to FIG. 4 and the description is not repeated herein.

Referring further to FIG. 13, by forming the buffer layer 1101 between the conductive layer 901 and the phase-change layer 1201, metal diffusion from the conductive layer 901 into the phase-change layer 1201 may be reduced or avoided. In some embodiments where the conductive layer 901 comprises TiN, the buffer layer 1101 prevents Ti atoms from diffusing into the phase-change layer 1201. By forming the buffer layer 1203 between the conductive layer 1205 and the phase-change layer 1201, metal diffusion from the conductive layer 1205 into the phase-change layer 1201 may be reduced or avoided. In some embodiments where the conductive layer 1205 comprises TiN, the buffer layer 1203 prevents Ti atoms from diffusing into the phase-change layer 1201. In some embodiments, by avoiding the metal contaminants in the phase-change layer 1201, read/write cycling endurance of the PCRAM structure 1207 may be improved. In some embodiments, the conductive layers 901 and 1205 comprise TiN and the buffer layers 1101 and 1203 comprise amorphous carbon. In other embodiments, the conductive layer 901 comprises Ru, the conductive layer 1205 comprises TiN, and the buffer layers 1101 and 1203 comprise amorphous carbon. In yet other embodiments, the conductive layer 901 comprises TiN, the conductive layer 1205 comprises Ru, and the buffer layers 1101 and 1203 comprise amorphous carbon.

FIG. 14 illustrates a cross-sectional view of a PCRAM cell 1400 in accordance with some embodiments. In some embodiments, the PCRAM cell 1400 may be similar to the PCRAM cell 800 illustrated in FIG. 13, with similar features of the PCRAM cell 1400 and the PCRAM cell 800 being labeled with similar numerical references and descriptions of the similar features are not repeated herein. In some embodiments, the PCRAM cell 1400 may be formed using process steps described above with reference to FIG. 8-13, but omitting the formation of the buffer material 207. In distinction with the PCRAM structure 1207 of the PCRAM cell 800 (see FIG. 13), the buffer layer 1203 is omitted in the PCRAM structure 1401 of the PCRAM cell 1400 and the PCRAM structure 1401 comprises the conductive layer 1205 in physical contact with the phase-change layer 1201. By forming the buffer layer 1101 between the conductive layer 901 and the phase-change layer 1201, metal diffusion from the conductive layer 901 into the phase-change layer 1201 may be reduced or avoided. In some embodiments, the metal diffusion from the conductive layer 1205 into the phase-change layer 1201 may be reduced or avoided by forming the conductive layer 1205 with a conductive material comprising metal elements with reduced diffusion coefficients. In such embodiments, the conductive layer 1205 may comprise Ru, W, WN, C, a combination thereof, a multilayer thereof, or the like.

FIG. 15 is a flow diagram illustrating a method 1500 of forming a PCRAM cell in accordance with some embodiments. The method 1500 starts with step 1501, where a dielectric layer (such as the dielectric layer 125 illustrated in FIG. 1) is formed over a first conductive feature (such as the conductive line 119 _(M) illustrated in FIG. 1) as described above with reference to FIG. 1. In step 1503, a bottom electrode (such as the conductive layer 201 illustrated in FIG. 2) is formed in the dielectric layer as described above with reference to FIGS. 1 and 2. In step 1505, a first buffer material (such as the buffer material 203 illustrated in FIG. 2) is formed over the bottom electrode and the dielectric layer as described above with reference to FIG. 2. In step 1507, a phase-change material (such as the phase-change material 205 illustrated in FIG. 2) is formed over the first buffer material as described above with reference to FIG. 2. In step 1509, a second buffer material (such as the buffer material 207 illustrated in FIG. 2) is formed over the phase-change material as described above with reference to FIG. 2. In step 1511, a top electrode material (such as the conductive material 209 illustrated in FIG. 2) is formed over the second buffer material as described above with reference to FIG. 2. In step 1513, the first buffer material, the phase-change material, the second buffer material, and the top electrode material are patterned to form a first buffer layer (such as the buffer layer 301 illustrated in FIG. 3), a phase-change layer (such as the phase-change layer 303 illustrated in FIG. 3), a second buffer layer (such as the buffer layer 305 illustrated in FIG. 3), and a top electrode (such as the conductive layer 307 illustrated in FIG. 3), respectively, as described above with reference to FIG. 3. In step 1515, a second conductive feature (such as the conductive via 121 _(M+1) illustrated in FIG. 4) is formed over the top electrode as described above with reference to FIG. 4. In some embodiments, step 1505 may be omitted. In other embodiments, step 1509 may be omitted. In yet other embodiments, steps 1505 and 1509 may be omitted.

FIG. 16 is a flow diagram illustrating a method 1600 of forming a PCRAM cell in accordance with some embodiments. The method 1600 starts with step 1601, where a dielectric layer (such as the dielectric layer 125 illustrated in FIG. 8) is formed over a first conductive feature (such as the conductive line 119 _(M) illustrated in FIG. 8) as described above with reference to FIG. 8. In step 1603, a bottom electrode (such as the conductive layer 901 illustrated in FIG. 9) is formed in the dielectric layer as described above with reference to FIGS. 8 and 9. In step 1605, the bottom electrode is recessed below a topmost surface of the dielectric layer to form a recess (such as the recess 1001 illustrated in FIG. 10) as described above with reference to FIG. 10. In step 1607, a first buffer layer (such as the buffer layer 1101 illustrated in FIG. 11) is formed over the bottom electrode in the recess as described above with reference to FIG. 11. In step 1609, a phase-change material (such as the phase-change material 205 illustrated in FIG. 11) is formed over the first buffer layer and the dielectric layer as described above with reference to FIG. 11. In step 1611, a second buffer material (such as the buffer material 207 illustrated in FIG. 11) is formed over the phase-change material as described above with reference to FIG. 11. In step 1613, a top electrode material (such as the conductive material 209 illustrated in FIG. 11) is formed over the second buffer material as described above with reference to FIG. 11. In step 1615, the phase-change material, the second buffer material, and the top electrode material are patterned to form a phase-change layer (such as the phase-change layer 1201 illustrated in FIG. 12), a second buffer layer (such as the buffer layer 1203 illustrated in FIG. 12), and a top electrode (such as the conductive layer 1205 illustrated in FIG. 12), respectively, as described above with reference to FIG. 12. In step 1617, a second conductive feature (such as the conductive via 121 _(M+1) illustrated in FIG. 13) is formed over the top electrode as described above with reference to FIG. 13. In some embodiments, step 1611 may be omitted.

In an embodiment, a device includes: a substrate; a first dielectric layer over the substrate; a bottom electrode extending through the first dielectric layer; a first buffer layer over the bottom electrode; a phase-change layer over the first buffer layer; a top electrode over the phase-change layer, wherein a width of the top electrode is greater than a width of the bottom electrode; and a second dielectric layer over the first dielectric layer, the second dielectric layer surrounding the phase-change layer and the top electrode.

In another embodiment, a device includes: a substrate; a first dielectric layer over the substrate; a conductive line in the first dielectric layer; a second dielectric layer over the conductive line and the first dielectric layer; a bottom electrode within the second dielectric layer, the bottom electrode being electrically connected to the conductive line, the bottom electrode having a first width; a first buffer layer over the bottom electrode, the first buffer layer having a second width; a phase-change layer over the first buffer layer, the phase-change layer having a third width greater than the first width; a top electrode over the phase-change layer, the top electrode having the third width; a third dielectric layer over the second dielectric layer, the third dielectric layer extending along sidewalls of the phase-change layer and sidewalls of the top electrode; a fourth dielectric layer over the third dielectric layer and the top electrode; and a conductive via within the fourth dielectric layer, the conductive via being electrically connected to the top electrode.

In yet another embodiment, a method includes: forming a first dielectric layer over a substrate; forming a first conductive feature in the first dielectric layer; forming a second dielectric layer over the first dielectric layer and the first conductive feature; forming an opening in the second dielectric layer, the opening exposing a topmost surface of the first conductive feature; filling the opening with a first conductive material to form a bottom electrode in the second dielectric layer; depositing a phase-change material over the second dielectric layer and the bottom electrode; depositing a first buffer material over the phase-change material; depositing a second conductive material over the first buffer material; performing a patterning process to remove a portion of the phase-change material, a portion of the first buffer material and a portion of the second conductive material and to expose a topmost surface of the second dielectric layer, a remaining portion of the phase-change material forming a phase-change layer, a remaining portion of the first buffer material forming a first buffer layer, a remaining portion of the second conductive material forming a top electrode; and forming a third dielectric layer over the second dielectric layer, the third dielectric layer surrounding the first buffer layer, the phase-change layer and the top electrode.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A device comprising: a substrate; a first dielectric layer over the substrate; a bottom electrode extending through the first dielectric layer; a first buffer layer over the bottom electrode; a phase-change layer over the first buffer layer; a top electrode over the phase-change layer, wherein a width of the top electrode is greater than a width of the bottom electrode; and a second dielectric layer over the first dielectric layer, the second dielectric layer surrounding the phase-change layer and the top electrode.
 2. The device of claim 1, wherein the first buffer layer and the bottom electrode have a same width.
 3. The device of claim 2, wherein a topmost surface of the first buffer layer is substantially level with a topmost surface of the first dielectric layer.
 4. The device of claim 1, wherein a width of the first buffer layer is greater than the width of the bottom electrode.
 5. The device of claim 4, wherein a topmost surface of the first buffer layer is above a topmost surface of the first dielectric layer.
 6. The device of claim 1, further comprising a second buffer layer between the phase-change layer and the top electrode, wherein the second dielectric layer surrounds the second buffer layer.
 7. The device of claim 1, wherein the first buffer layer comprises amorphous carbon, W, WN, TaN, Ru, graphene, MoS₂, or WS₂.
 8. A device comprising: a substrate; a first dielectric layer over the substrate; a conductive line in the first dielectric layer; a second dielectric layer over the conductive line and the first dielectric layer; a bottom electrode within the second dielectric layer, the bottom electrode being electrically connected to the conductive line, the bottom electrode having a first width; a first buffer layer over the bottom electrode, the first buffer layer having a second width; a phase-change layer over the first buffer layer, the phase-change layer having a third width greater than the first width; a top electrode over the phase-change layer, the top electrode having the third width; a third dielectric layer over the second dielectric layer, the third dielectric layer extending along sidewalls of the phase-change layer and sidewalls of the top electrode; a fourth dielectric layer over the third dielectric layer and the top electrode; and a conductive via within the fourth dielectric layer, the conductive via being electrically connected to the top electrode.
 9. The device of claim 8, wherein the bottom electrode and the top electrode comprise different conductive materials.
 10. The device of claim 8, wherein the bottom electrode and the top electrode comprise a same conductive material.
 11. The device of claim 8, further comprising a second buffer layer between the phase-change layer and the top electrode, the second buffer layer having the third width.
 12. The device of claim 8, wherein the first width is less than the second width.
 13. The device of claim 8, wherein the first width is equal to the second width.
 14. The device of claim 8, wherein the first buffer layer comprises amorphous carbon, W, WN, TaN, Ru, graphene, MoS₂, or WS₂.
 15. A method comprising: forming a first dielectric layer over a substrate; forming a first conductive feature in the first dielectric layer; forming a second dielectric layer over the first dielectric layer and the first conductive feature; forming an opening in the second dielectric layer, the opening exposing a topmost surface of the first conductive feature; filling the opening with a first conductive material to form a bottom electrode in the second dielectric layer; depositing a phase-change material over the second dielectric layer and the bottom electrode; depositing a first buffer material over the phase-change material; depositing a second conductive material over the first buffer material; performing a patterning process to remove a portion of the phase-change material, a portion of the first buffer material and a portion of the second conductive material and to expose a topmost surface of the second dielectric layer, a remaining portion of the phase-change material forming a phase-change layer, a remaining portion of the first buffer material forming a first buffer layer, a remaining portion of the second conductive material forming a top electrode; and forming a third dielectric layer over the second dielectric layer, the third dielectric layer surrounding the first buffer layer, the phase-change layer and the top electrode.
 16. The method of claim 15, further comprising before depositing the phase-change material, depositing a second buffer material over the second dielectric layer and the bottom electrode.
 17. The method of claim 16, wherein performing the patterning process further comprises removing a portion of the second buffer material, a remaining portion of the second buffer material forming a second buffer layer, the second buffer layer and the first buffer layer having a same width.
 18. The method of claim 15, further comprising recessing the bottom electrode below the topmost surface of the second dielectric layer to form a recess.
 19. The method of claim 18, further comprising filling the recess with a second buffer material to form a second buffer layer in the recess, a width of the second buffer layer being less than a width of the first buffer layer.
 20. The method of claim 15, further comprising: forming a fourth dielectric layer over the third dielectric layer and the top electrode; and forming a second conductive feature in the fourth dielectric layer, the second conductive feature being in electrical contact with the top electrode. 